Mashrekur Rahman, Jonathan Frame , Jimmy Lin, Grey Nearing 

We used Natural Language Processing (NLP) to assess topic diversity in all research articles (∼75,000) from eighteen water science and hydrology journals published between 1991 and 2019. We found that individual water science and hydrology research articles are becoming increasingly interdisciplinary in the sense that, on average, the number of equally-common topics represented in individual articles is increasing. This is true even though the body of water science and hydrology literature as a whole is not becoming more topically diverse. These findings suggest that the National Research Council’s (1991) recommendation to increase multidisciplinarity of hydrological research has been followed. Topics with the largest increases in popularity were Climate Change Impacts, Water Policy & Planning, and Pollutant Removal. Topics with the largest decreases in popularity were Stochastic Models and Numerical Models. At a journal level, Water Resources Research, Journal of Hydrology, and Hydrological Processes are the three most topically diverse journals in the discipline. We also identified topics that are becoming increasingly isolated, and which could potentially benefit from integrating more with the wider hydrology discipline.